Re: Spitfire
Armament
Spitfire at the Wright-Patterson Air Force Museum
Due to a shortage of Brownings, which had been selected as the new standard rifle calibre machine gun for the RAF in 1934, early Spitfires were fitted with only four guns, with the other four fitted later.[95] Early tests showed that while the guns worked perfectly on the ground and at low altitudes, they tended to freeze at high altitude, especially the outer wing guns, because the RAF's Brownings had been modified to fire from an open bolt. While this prevented overheating of the cordite used in British ammunition, it allowed cold air to flow through the barrel unhindered.[96] Supermarine did not fix the problem until October 1938, when they added hot air ducts from the rear of the wing-mounted radiators to the guns, and bulkheads around the gunbays to trap the hot air in the wing. Red fabric patches were doped over the gun ports to protect the guns from cold, dirt, and moisture until they were fired.[97]
The decision on the arming of the Spitfire (and the Hurricane) is told in Captain C. H. Keith's book I Hold my Aim. Keith held various appointments with the RAF dealing with designing, development and technical policy of armament equipment. He organised a conference, with Air Commodore Tedder in the chair, on 19 July 1934. He says "I think it can be reasonably contended that the deliberations of that conference made possible, if not certain, of the winning of the Battle of Britain, almost exactly six years later".[98] At that meeting, scientific officer Captain F.W. 'Gunner' Hill presented charts based on his calculations showing that future fighters must carry no less than eight machine-guns, each of which must be capable of firing 1,000 shots a minute. Hill's assistant in making his calculations had been his 14-year-old daughter.
Even if the eight Brownings worked perfectly, pilots soon discovered that they were not sufficient to destroy larger aircraft. Combat reports showed that an average of 4,500 rounds were needed to shoot down an enemy aircraft. In November 1938, tests against armoured and unarmoured targets had already indicated that the introduction of a weapon with a calibre of at least 20 mm was urgently needed.[99] A variant on the Spitfire design with four 20 mm Oerlikon cannon had been tendered to specification F37/35, but the order for prototypes had gone to the Westland Whirlwind in January 1939.[100]
In June 1939, a Spitfire was fitted with a drum-fed Hispano in each wing, an installation that required large blisters on the wing to cover the 60-round drum. The cannon suffered frequent stoppages, mostly because the guns were mounted on their sides to fit as much of the magazine as possible within the wing. In January 1940, P/O George Proudman flew this prototype in combat, but the starboard gun stopped after firing a single round, while the port gun fired 30 rounds before seizing.[97] If one cannon seized, the recoil of the other threw the aircraft off aim.
Nevertheless, 30 more cannon-armed Spitfires were ordered for operational trials, and they were soon known as the Mk IB, to distinguish them from the Browning-armed Mk IA; they were delivered to No. 19 Squadron beginning in June 1940. The Hispanos were found to be so unreliable that the squadron requested an exchange of its aircraft with the older Browning-armed aircraft of an operational training unit. By August, Supermarine had perfected a more reliable installation with an improved feed mechanism and four .303s in the outer wing panels. The modified fighters were then delivered to 19 Squadron.[97]